How to go about modelling this roof shape in Blender?
I guess you need to switch to another way of modeling in Blender (compared to some other 3D softwares), you could start from the shape you show but maybe the best is to start from scratch.
Create a cube, subdivide, pull the top edge:

Create an additional edge loop on the left:

Extrude the side walls:

Cut an horizontal edge:

Pull the vertex:

Rework a bit (the knife tool is useful, K to activate, W to cut through, C to cut perpendicularly, also bevel for the top, CtrlB):

Since latest versions you have the Extrude Manifold tool (AltE) that allows to work a bit like Sketchup or others, but you often need to give some corrections afterwards to keep a good topology (good doesn't meaning a lot, it all depends on your needs of course):